Coating process using radiation curable compositions

Abstract: A process is provided whereby radiation curable compositions comprising N-vinylformamide and an oligomer selected from the group consisting of epoxy-acrylate resins, polyester acrylate resins, polyurethane acrylate resins, acrylic acrylate resins, silicone acrylate resins, unsaturated polyester resins, maleate ester resins, vinyl ether resins and mixtures thereof are applied to the surface of a substrate and subsequently exposed to a radiation source until an adherent dry polymerized film is formed on the substrate. These compositions are useful for a wide range of applications including pigmented and unpigmented coatings, printing inks, adhesives and the like.

Description: FIELD OF THE INVENTION The present invention relates to the formulation and applications of radiation curable compositions suitable for use as pigmented or unpigmented coatings, printing inks, adhesives and other applications. EP-A-0 559 135 relates to a radiation curable formulation comprising: (a) an oligomer selected from the group consisting of epoxy-acrylate resins, polyester acrylate resins, polyurethane acrylate resins and mixtures thereof; and (b) N-vinyl formamide. BACKGROUND OF THE INVENTION Radiation curable acrylic oligomers are usually comparatively high viscosity species and require to be blended with a monomer in order to produce a radiation curable formulation of the appropriate viscosity for any particular application. Such oligomers generally fall into three broad groups of resin, namely epoxy-acrylates, polyester acrylates and polyurethane acrylates. The epoxy-acrylates include the B-hydroxy esters which are generated by the reaction of acrylic acid or methacrylic acid with an epoxy resin or epoxy-novolak resin. The polyester acrylates consist of polyesters which have been esterified with acrylic acid to yield a polyester with acrylate ester terminal groups, using well established esterification techniques. The polyurethane acrylates consist of reaction products of a hydroxy-containing acrylate ester, usually 2-hydroxyethyl acrylate or hydroxy propyl acrylate with an isocyanate prepolymer. The monomers which are blended with the above acrylic oligomers in order to yield a practical radiation curable formulation in the presence of a suitable photoinitiator fall into three groups defined by functionality, and may be mono-, di- or poly-functional. Poly-functional monomers, usually with a functionality of 3 or 4, generally consists of acrylate esters of tri or tetra-functional alcohols. Commonly used materials include glycerol triacrylate, trimethylol propane triacrylate, trimethylol ethane triacrylate, pentaerythritol tetracrylate, together with the acrylates of the ethoxylates or propoxylates of the above alcohols. Di-functional monomers consist usually of the acrylate esters of ethylene glycol or propylene glycol and their oligomers, with tripropylene glycol diacrylate being especially preferred, diacrylates of longer chain alcohols such as hexanediol diacrylate and acrylate esters of cycloaliphatic diols such as the cyclohexane diols. Mono functional monomers consist of the acrylate esters of mono functional alcohols such as octanol, nonanol, decanol, dodecanol, tri decanol and hexadecanol both in their linear and branch chain forms. Also included are cyclohexyl acrylate and its alkyl derivatives such as t-butyl cyclohexyl acrylate, tetrahydro furfuryl acrylate. N-vinylpyrrolidone has also been used as a mono-functional monomer. Styrene is used in certain formulations but is not widely used in this technology due to triplet quenching. High functionality monomers give rapid cure speeds and high cross-link density, leading to films of high hardness and tensile strength with excellent chemical resistance. The films however suffer from reduced adhesion. Such monomers exhibit comparatively poor ability to reduce the working viscosity of the oligomer, due to the higher initial viscosity associated with the monomers themselves. Mono functional monomers, conversely, give slow cure speeds and low cross-link density, leading to cured films of lower hardness, tensile strength, and with reduced chemical resistance. Such monomers give films with improved elongation and improved adhesion, and the monomers show a considerably increased capacity to reduce the working viscosity of the oligomer. Prucnal, et al., U.S. Patent 3,874, 906 teach a method of applying and curing a polyester-acrylate containing coating composition comprising adding N-vinylpyrrolidone to the composition and subsequently applying the composition to a substrate and subjecting it to actinic light to cure. Lorenz, et al., U.S. Patent 4,129,709 disclose a coating composition comprising an oligomer produced by reacting polytetrahydrofuran with a diisocyanate, N-vinyl-2-pyrrolidone and an acrylic acid ester having a boiling point of at least 200°C at 760mm Hg. Priola, et al., U.S. Patent 4,348,427 teach a method of coating surfaces by applying to the surface to be coated, a mixture composed by at least one compound such as an epoxy-acrylate resin, a polyester alpha-omega-acrylate resin, an unsaturated polyester resin, or a urethane-acrylate resin and by at least one unsaturated compound of the amide, lactam, piperidone and urea classes and subsequently irradiating the coated surface with radiation in the range of 200 to 400nm. SUMMARY OF THE INVENTION The present invention is an improved radiation curable composition comprising an oligomer selected from the group consisting of acrylic acrylate resins, silicone acrylate resins, unsaturated polyester resins, maleate ester resins, vinyl ether resins and mixtures thereof, and N-vinylformamide. A photoinitiator may optionally be added to the composition prior to applying the composition to a substrate. Optionally, the composition may contain other reactive monomers such as mono-, di-, or poly-functional acrylic esters, or other vinyl compounds. The present invention also covers a process for coating a substrate surface using the subject composition. In accordance with this process, the composition, optionally containing a photoinitiator, is applied to the substrate surface and is subsequently exposed to a radiation source until an adherent dry polymerized film is formed on the substrate. We have discovered that the use of N-vinylformamide as a monomer in radiation curable coating formulations results in a superior coating material compared to prior art formulations. Compared to acrylate types of monomers, N-vinylformamide exhibits superior capacity to reduce the working viscosity of the oligomer, and is especially useful with polyurethane acrylate containing formulations with which viscosity reduction is particularly difficult with monomers traditionally used with such systems. Additionally, films cured with N-vinylformamide demonstrate excellent hardness, chemical resistance, flexibility, scuff resistance and scratch resistance. DETAILED DESCRIPTION OF THE INVENTION We have discovered improved radiation curable compositions which are useful for forming hard protective polymer films on substrate surfaces. The composition is an improvement over the prior art in that when N-vinylformamide (NVF) monomer is blended with certain oligomer systems, the resultant formulation exhibits improved properties for applications such as protective and decorative coatings, resinous binders for pigmented inks, adhesive materials. We have found that NVF is an effective reactive diluent for radiation curable oligomer systems in that it exhibits low vapor pressure, good solvency, and rapid cure under standard photocure conditions to hard, flexible, clear, colorless coatings. Additionally, NVF significantly reduces working viscosities relative to traditionally used acrylate monomers, particularly in difficultly reducible polyurethane acrylate oligomer systems. Cured polymers made with NVF exhibit improved odor, harder films, good chemical resistance, flexibility, scuff resistance and scratch resistance. A further advantage of using NVF is that, since NVF is a secondary amide, the amide hydrogen is available as a reactive site for secondary curing reactions. Consequently, under proper conditions, the NVF will combine with species normally reactive with active hydrogen compounds to produce a separate non-volatile induced cure subsequent to or concurrent with free radical polymerization through the vinyl bond. Such secondary curing reactions are characteristic of so-called dual cure systems, and can be useful in modifying the finished properties of the film. The formulation is made by mixing NVF with an appropriate resin. The NVF is typically added in a concentration of between about 2 and 40 wt% and preferably between 5 and 25 wt% based on the total composition. The resins useful for such compositions include acrylic acrylate resins, silicone acrylate resins, unsaturated polyester resins, maleate ester resins and vinyl ether resins. Optionally, the composition may contain other reactive species such as the aforementioned acrylic monomers consisting of mono-, di-, or polyfunctional acrylic esters of suitable low molecular weight alcohols, acrylic esters of ethoxylated or propoxylated alcohols, diols, and triols; or other vinyl monomers. The resultant acrylate oligomer/NVF compositions are optionally mixed with a conventional photoinitiator used in this technology, which include for example, benzophenone, benzoin ethers and related species, as well as cationic photoinitiators. Monomers including t-amino groups may be used in certain formulations as activators for the photoinitiator. The use of a photoinitiator is optional depending upon the radiation source used for curing. The composition, containing the photoinitiator, is applied to the surface of a substrate and subsequently exposed to a radiation source until an adherent dry polymerized film is formed on the substrate. Sources of radiant energy appropriate for initiating cure of the formulations have been described extensively in the literature and are well known to those skilled in the art. These include various sources of particulate and non-particulate radiation producing wavelengths generally less than 700 nanometers. Especially useful is actinic radiation in the 180-440 nm range which can be conveniently obtained by use of one of several commercially available ultra-violet sources specifically intended for this purpose. These include low, medium and high pressure mercury vapor lamps, He-Cd and Ar lasers, xenon arc lamps, etc. Photoinitiator systems having a corresponding sensitivity to light in this wave band are normally incorporated into the formulation and upon irradiation lead to the formation of reactive species capable of initiating free radical polymerization. Similarly, free radical polymerization may be induced by exposure of the formulation to an electron beam without the use of a photoinitiator. Equipment capable of generating a curtain of electrons with energies between 150 and 300 KeV is particularly suitable for this purpose and its use is well documented in the literature. The composition which preferably has a viscosity between 3.25 and 1.00 Pas (32.5-10.0 poise) is useful for placement on a wide range of substrates including paper, rigid and flexible plastics, metallic substrates, cement, glass, asbestos products and wood wherein N-vinylformamide may be incorporated into the radiation-curable formulation in order to reduce working viscosity and/or promote some other useful property such as cure speed, cured film adhesion, chemical resistance and hardness. In particular, it is anticipated that a useful field of application for the invention will include formulation categories that have typically utilized N-vinyl-2-pyrrolidone, and where N-vinylformamide may be advantageously employed as a substitute for N-vinyl-2-pyrrolidone. Examples of formulation categories include, but are not limited to, the following: overprint varnishes for paper and board; screen, letterpress, flexographic, and gravure printing inks; stereolithography baths; pressure-sensitive and assembly adhesives; vinyl floor coatings; pigmented and unpigmented wood finishes; coatings for optical fiber; waterborne spray-applied coatings; base and top coatings for rigid and flexible plastics; etch and solder photomasks.

Claims: A process for coating a substrate comprising applying to said substrate a curable composition which composition comprises N-vinylformamide and an oligomer selected from the group consisting of acrylic acrylate resins, silicone acrylate resins, unsaturated polyester resins, maleate ester resins, vinyl ether resins and mixtures thereof; and exposing the coated substrate to a radiation source until an adherent dry polymerized film is formed on the substrate. The process of Claim 1 wherein said coating composition contains a photoinitiator. The process of Claim 1 wherein said coating composition contains between 2 and 40 wt% N-vinylformamide. The process of Claim 1 wherein said coating composition also contains a mono-, di-, or poly-functional vinyl or acrylic monomer. The process of Claim 1 wherein said curable composition has a viscosity of between about 3.25 and 1.00 Pa·s (32.5 and 10.0 poise). The process of Claim 2 wherein said photoinitiator is a free radical photoinitiator. The process of Claim 6 wherein said free radical photoinitiator is benzophenone or benzoin ether. The process of Claim 2 wherein said composition also contains a monomer which acts as an activator for the photoinitiator. The process of Claim 1 wherein said radiation source produces wavelengths less than 700 nm. The process of Claim 1 wherein said radiation source produces actinic radiation having wavelengths from 180-440 nm.

Inventors: CORNFORTH DAVID ARTHUR, HOWEL DAWN, RENZ WALTER LOUIS, CORNFORTH, DAVID ARTHUR, HOWEL, DAWN, RENZ, WALTER LOUIS
Applicants: AIR PROD & CHEM, AIR PRODUCTS AND CHEMICALS, INC.
Application year: 1994
Main classification: C09D4
IPC codes: C08F290, C09D4, B05D3, B05D7